You will have to weld in your own anchor points.
There are no captive nuts installed in the parcel tray for a child seat, nor are there any reinforced sections suitably placed.
The process is straightforward, remove rear seat, remove parcel tray, install the anchor points in the appropriately accessible location (directly behind the centre of the headrest is the widely accepted location), refit the parcel tray, drill through and affix the anchor point.
Australian standard safety requirements for the child safety anchor points are generally a square piece of 1/4" plate, minimum of 4" side length, with the appropriate high tensile bolt of 7/16"UNF minimum grade 8.8 to anchor the capsule.
Your location and legal requiremens may vary.
here's an excerpt from our VSB:(LK6)
mounting plates must have length times width dimensions of at least:
100mm x 75mm for vehicles up to 1150 kg tare mass, or
100mm x 100mm for vehicles greater than 1150 kg and up to 1600 kg tare mass; or
120mm x 120mm for vehicles greater than 1600 kg tare mass.
The lower underfloor mounting plates must be at least 120mm x 45mm. Mounting plates of equivalent area may be used.
Mounting plates must be free of sharp edges or protrusions and the mating surface edges must be rounded to reduce the likelihood of any guillotine action occurring.
Here's a 2.5-16 parcel tray (dec 1989 build)
1987 201.034 - € spec 5sp. Blauschwarz- Hermann.
1992 124.051 - 6sp man. Iceblau - Gretel
1992 124.090 - 4sp auto Malachit Grün - Beatrix
1987 107.048 - 4sp auto Signalrot - Stella
2012 212.074 - 7sp mct-speedshift Diamantweiß - Klaus
numerous other non-MB's
MB exit lounge: 1989 201.035, 1987 201.029, 1987 126.039, 1972 107.023